1. Field of the Invention
The present invention relates to signal processing. More specifically, the present invention relates to a method for automatic clutter elimination which uses a variety of techniques for performing different levels of clutter elimination from different signals.
2. Background Information
In the signal processing art, it is typical that raw signals will be received in a form wherein a signal which is desired to be processed and noise (also known as clutter), are contained within the same raw signal. Typically, in these circumstances and in certain prior art systems, filtering has been performed upon the raw signal in order to remove the clutter. In certain prior art electronic implementations of signal processing devices, including those in use in ultrasonic color Doppler imaging, filters such as wall or high-pass filters have been used to remove low frequency information from the signal indicating stationary or very slow moving objects, such as those due to tissue or slow-moving blood flow. In these techniques, Discrete Fourier Transform (DFT), Infinite Impulse Response (IIR), and Finite Impulse Response (FIR) filters have been used to process the raw signal data into a form which may be used for various purposes, including color Doppler imaging. In another technique, clutter is removed from the signal using successively higher order basis functions (e.g., the Legendre functions) until the signal desired to be processed becomes apparent. This is described in the Applicants' previous application entitled "Parametric Clutter Elimination," filed on Apr. 10, 1992, assigned U.S. Ser. No. 07/867,038, now U.S. Pat. No. 5,228,009, which has been assigned to the assignee of the present invention.
In prior art systems, the same level of filtering or the same order of basis functions is removed for all signals which are being processed. For example, in the case of ultrasonic color Doppler imaging wherein the processed signals are displayed in various colors representing motion in the body, the same level of filtering (whether by prior art methods of filtering or Applicants' Parametric Clutter Elimination techniques and apparatus described in U.S. Pat. No. 5,228,009), the same level of filtering is applied for an entire group of signals (e.g., an entire frame in a region of interest). This approach, however, suffers from several defects.
In the case of color Doppler imaging wherein a set of signals from a region of interest (ROI) in a subject are used to form a video frame for display, higher levels of filtration may result in the distortion or elimination of signal data for areas in which there is slow moving blood flow by severely biasing the signal. For example, when viewing arterial and venous blood flow simultaneously in the same ROI, heavy filtering may be required for the arterial blood flow in order to remove clutter due to motion of the arterial wall. For arterial blood flow, using Applicants' U.S. Pat. No. 5,228,009, a linear or quadratic component may be desired to be removed from the raw signal data. For venous blood flow using this technique, in contrast, a mean may be desired to be removed from the raw signal. Thus, within a single region of interest in the case of ultrasonic examination, different levels of filtering are desirable for the various signals to generate the frame. Prior art methods limit the user to selecting one type of filtering which is performed upon all scans of each ROI. This poses substantial problems because signals representing venous blood flow may be severely biased to an undetectable level by the high level of filtering applied to the arterial blood flow. Thus, a fixed setting of the filter for an entire frame or ROI is undesirable.
Yet another defect of prior art signal processing methods and apparatus for removing clutter is that, typically, the level of filtration is controlled by the user via selection using a selector dial or similar input apparatus (e.g., icon selected on a display screen). For example, if the user requires a single level of filtering (e.g., removing a mean from the signal, as in Applicants' prior U.S. Pat. No. 5,228,009), then that level's filtering is applied entirely across the frame. As previously described, this is unsatisfactory. Moreover, user intervention is required to select the appropriate level of filtering. A technician being familiar with ultrasonic operating concepts thus may, depending upon operating circumstances, adjust the level of filtering to an appropriate level. However, an inexperienced technician may select an inappropriate filtering level. Thus, prior art methods of filtering signals, especially for use in ultrasonic imaging apparatus, suffers from several disadvantages.